Optical fibers and waveguides in their simplest construction consist of a so-called core material through which the majority of the optical information passes and, surrounding the core, a cladding material which transmits some of the light but whose principal function is to restrict the optical information to the core region of the construction.
Core materials have been either siliceous (glass) or organic polymer in nature. While certain advantages, such as outstanding flexural strength, ease of processing, and facile connectorization attend organic polymer cores, glass cores are virtually unchallenged in their ability to transmit optical information with a low degree of attenuation or loss. Therefore, considerable effort has been made in the art to utilize glass cores despite obvious drawbacks such as brittleness, moisture sensitivity, and extreme processing conditions.
Effective cladding materials exhibit low refractive indices and low moisture vapor transmission rates. Fluorinated polymer claddings have been described to meet these criteria on both organic polymer cores (for example, in U.S. Pat. Nos. 4,505,543; 4,544,235; 4,557,562; 4,660,923; 4,687,295; and 4,836,642) and glass cores (for example, in Eur. Pat. Appl. No. 128,516; Eur. Pat. Appl. No. 239,935; U.S. Pat. Nos. 4,720,428 and 4,804,246.). A problem with these fluorinated polymer systems, however, is that they are applied to the core material either from solution which can be polluting and requires complete outgassing of even the last traces of solvent for optimum performance or are melt extruded onto the core which can be very physically damaging to the surface of the core.
An innovation in the fiber optics industry was put forth by Skutnik in U.S. Pat. No. 4,511,209 describing so-called "hard clad silica" (HCS) fibers that were cured by ultraviolet light. Earlier approaches to UV curable cladding/buffer materials had stressed that the cladding or primary coating should be very elastomeric and possess a low modulus, while the buffer or secondary protective coating should be a tough, high modulus material. These precepts for so-called "plastic clad silica" (PCS) fibers usually involved very low modulus silicone cladding materials and are described, for example, by L. L. Blyler, Jr., et al., "Polymers for High Technology", ACS Symp. Ser. 346, edited by M. J. Bowden and S. R. Turner, published by the American Chemical Society: Washington, D.C., Chapter 34, 1987. In contrast, HCS fibers (further described by W. B. Beck and M. H. Hodge, "Laser Focus/Electrooptics", pp. 90-96 (1984) and by B. J. Skutnik, et al., Mat. Res. Symp. Proc., 1987, 88, 27) feature a hard polymer cladding that is chemically bonded to the glass core. HCS fibers feature high core-to-clad diameter ratios which facilitate physical coupling of fibers and reduce microbending losses. Additionally, the chemical bond between core and clad increases fiber strength and minimizes the corrosive effects of water. HCS UV curable cladding compositions of U.S. Pat. No. 4,511,209 comprise (i) at least 50 weight percent of a highly fluorinated monofunctional acrylate with a refractive index below 1.38; (ii) a polyfunctional acrylate crosslinking agent; (iii) a mono or polyfunctional thiol synergist (some of which are alkoxysilane-functional and provide a covalent bond to the glass core); and (iv) a photoinitiator.
There are several disadvantages with the above system, perhaps the greatest being low viscosity. The major component, the fluorinated acrylate monomer, is either a liquid with a water-like viscosity or a solid that requires a solvent or melting in order to coat. Coating such a low viscosity liquid creates essentially a cylindrical column of liquid. This is an unstable situation and leads to a common problem in the industry known as "beading" of the coating which is totally unacceptable for a cladding material. Another disadvantage with the HCS fibers of U.S. Pat. No. 4,511,209 is the adhesion promoting alkoxysilane "synergist" requires outgassing of an alcohol which is created in the reaction with a silanol group on the core. Such outgassing can create voids which are detrimental to optical performance. Furthermore, the alkoxysilane-silanol condensation reaction leads to an overall increase in the densification of the cladding, i.e., a net shrinkage in volume occurs from about 25% to as much as 50% in volume at the critical core/cladding interface. This shrinkage is detrimental to adhesion and to the physical integrity of the cladding material.
An effort to improve the viscosity characteristics of a UV curable cladding composition was made by Kamei and Umaba in Eur. Pat. Appl. 243,605. This patent application teaches use of (i) a fluorine-containing polymer, (ii) a fluorine-containing monomer, (iii) a non-fluorine-containing monomer, and (iv) a polyfunctional crosslinking monomer. The increased viscosity of the overall formulation is accomplished by the presence of the fluorinated polymer component. A major problem with this system, however, is that low refractive index is sacrificed somewhat by dilution with relatively high refractive index, non-fluorinated monomers. The non-fluorinated components are required largely because fluorinated monomers themselves are relatively poor solvents, even for relatively low concentrations of polymers derived from them.
Eur. Pat. Appl. 257,863 relates to UV curable cladding compositions comprising low viscosity components.